1. Field of Invention
The present invention relates to digital signal correlation techniques. In particular, the present invention relates to a method and apparatus for reducing the time required to acquire a Global Position System (“GPS”) signal.
2. Description of the Background Art
The process of measuring a global positioning system (GPS) signal begins with a procedure to search for the GPS signal in the presence of noise by attempting a series of correlations of the incoming signal against a known pseudo-random noise (PRN) code. The search process can be lengthy, as both the frequency of the signal and the time-of-arrival delay are unknown. To find the signal, receivers traditionally conduct a two dimensional search, checking each delay possibility at a variety of possible frequencies. To test for the presence of a signal at a particular frequency and delay, the receiver is tuned to the frequency, and the incoming signal is correlated with the known PRN code delayed by an amount corresponding to an estimated time of arrival. If no signal is detected, the search continues to the next delay possibility, and after all delay possibilities are checked, continues to the next frequency possibility. Each individual correlation is performed over one or more milliseconds in order to allow sufficient signal averaging to distinguish the signal from the noise. Because many thousands of frequency and delay possibilities are tested, the overall acquisition process can take tens of seconds. This search technique is repeated for the signal from each satellite needed to calculate a position and time result, e.g., five satellite signals.
In an effort to reduce the search time required to achieve a position location solution, various techniques have been explored that provide aiding information to the GPS receiver. The aiding information generally provides satellite ephemeris as well as an estimate of the receiver's position. The aiding information is generally coupled to the GPS receiver via a wireless network.
In U.S. Pat. No. 6,133,874, the GPS receiver acquires a GPS signal from a first GPS satellite using a conventional search technique as described above. A first pseudorange to the first GPS satellite is computed using this conventional technique. The GPS receiver then uses the aiding information (e.g., the satellite position, time of day and receiver approximate position) in combination with the first pseudorange to estimate the pseudorange to the next satellite. This combination of information enables the search range (time range) to be substantially reduced for each additional satellite signal. The signals from each satellite are sequentially processed in this manner until enough satellite signals are received to compute the position of the GPS receiver.
In U.S. Pat. No. 6,070,078, a GPS receiver obtains ephemeris information and a server calculates predicted PRN code shift positions based on a known location and the ephemeris data. A reduced functionality GPS receiver within a cellular telephone receives a time reference and then searches a limited number of PRN code shift positions for each of a plurality of GPS satellites based on the predicted PRN code shift positions. If a time reference is not transmitted to the reduced functionality GPS receiver, a PRN code shift position for a first satellite signal is measured by searching all possible PRN code shift positions. The measured PRN code shift position for the first satellite signal is then used to reduce the range of possible PRN code shift positions for remaining satellite signals.
The technique of U.S. Pat. No. 6,133,874 requires a pseudorange to be computed with respect to the first satellite. In a low signal to noise ratio environment, such a computation may require an excessive amount of time to achieve as many correlations as required to accurately compute the pseudorange. In addition, microprocessor time must be spent to compute the pseudorange. The technique of U.S. Pat. No 6,070,078 requires a time reference to reduce the search range for GPS signals. If a time reference is unavailable, the technique of U.S. Pat. No. 6,070,078 requires all delay possibilities for a first satellite signal to be searched. Again, in a low signal to noise ratio environment, such a computation may require an excessive amount of time to search all delay possibilities in order to accurately acquire the first satellite signal.
Therefore, there is a need in the art for a method and apparatus that reduces the amount of time required to acquire GPS satellite signals.